The field of the invention is that of multiplexed data communication by radio, especially (but not, exclusively) using the TDMA (Time Division Multiple Access) technique.
The invention finds an advantageous application in cellular mobile radio systems, for example those to the GSM (Global System for Mobile Communications) standard. The invention has been specifically developed for PMR (Professional Mobile Radio) systems to the TETRA (Trans European Trunk Radio System) standard intended for networks used by emergency services (fire, police), taxi fleets, etc.
In this type of system calls that have been set up (i.e. are in stable mode) are conveyed by traffic channels (TCH). Each traffic channel is assigned a transmission frequency (or a set of frequencies in the GSM system, which uses the principle of frequency hopping). Several TCH share the same traffic frequency by time-division multiplexing between a plurality of mobile stations (typically four under the TETRA standard or eight at present under the GSM standard). Each mobile station communicates on one time slot of the multiplex.
Each call can convey either speech signals digitally coded at a low bit rate (below 16 kbit/s) and generally organized into "frames" or data as such, whether in circuit-switched mode (continuous transmission) or in packet-switched mode (continuous connection or using packets with their address in a header). Speech transmission is either full-duplex or half-duplex. In the latter case the mobile user must usually press a "push to talk" key to activate the up channel (from the mobile terminal to the radio communication infrastructure) in place of the down channel.
In a system like the TETRA system a call is assigned a time slot on an up frequency and a down frequency throughout the duration of a call (message trunking) or during transmission (transmission trunking).
It may be necessary temporarily to exceed the possible transmission rate on a traffic channel.
For example, it may be necessary to transmit signalling information relating to management of the traffic channel in question. The signalling information comprises instructions or parameters associated with network operation, for example.
A first known way to solve this problem is to increase the number of time slots assigned to the application. This technique is not the optimum, however, in that the secondary information sources transmit intermittently and at random and the reserved additional channels would be highly under-used.
Another known way to solve this problem, used in the GSM system among other things to provide the signalling data transmission service, is to use a "frame stealing" technique whereby a speech or data frame is replaced with a signalling message. The signalling messages are preceded by a stealing flag and the receiver has means for noting the presence of the flag and therefore of recognizing that the frame received contains signalling. It can then carry out the operations to which the signalling message refers and "reconstruct" the missing traffic frame in the appropriate manner, according to whether it comprises speech or data.
This technique also has various drawbacks, however.
First of all, frame stealing is usually arbitrary i.e. authoritarian and without discrimination, being applied as and when the need arises. This causes deterioration of traffic information transmission quality.
Secondly, frame stealing is currently restricted to channel-associated signalling because of the deterioration in quality it causes. This technique could be applied to other services, however.
International patent application WO 91/02436 discloses the transmission of signalling data in a traffic channel set up when temporary cessation of activity is detected on said channel. This document also discloses triggering of a time delay in response to reception of a signalling message to establish a maximum duration on expiry of which the message must be transmitted. This time-delay depends on the urgency of the message. If a time-delay has expired and no cessation of activity has been detected on the traffic channel the message is transmitted anyway and disrupts the speech/data signal conveyed on the channel.
This document therefore provides only two levels of relative importance in respect of the traffic channel (signal transmission/cessation of signal transmission) and suggests the association with each signalling message to be transmitted of a time-delay which is dependent on the urgency of the message to be transmitted. The time-delay mechanisms described are used to define a null, predetermined or infinite maximum time-delay on expiry of which said signalling message must be transmitted. Assuming that more than two levels of relative importance are defined for the signal transmitted on the traffic channel, one of these levels is of no utility in the context of this document as there is merely associated initially with each signalling message a time-delay which implicitly defines only two levels of relative importance (zero time-delay and infinite time-delay). This document does not disclose any comparison of the respective levels of importance of the speech/data signal and the signalling message.
The article "Blank and burst transmission of data over active speech channels" by WONG et al. published in Electronics Letters, vol. 24, n.degree. 11, 26 May 1988, describes a method of classifying speech segments whereby they can be omitted or not without disturbing the speech signal containing them. The document concerns the transmission of data on a traffic channel conveying a speech signal. The method is based on measuring a signal/segment reconstruction noise ratio.